/**
*      author: Y_Yao
*      created: 2025-05-10 20:13:25
**/

#include<bits/stdc++.h>

#define int LL
using namespace std;
using LL = long long;
const LL LLINF = 9e18;
const int INF = 0x3f3f3f3f;
const int N = 3e5 + 7;
int n;
int a[N];
int sum[N];

void solve()
{
    cin >> n;
    for(int i = 1;i <= n;i++)
    {
        cin >> a[i];
    }

    for(int i = n;i >= 1;i--)
    {
        sum[i] = sum[i + 1] + a[i];
    }

    int ans = 0;
    for(int i = 1;i <= n;i++)
    {
        ans += a[i] * (sum[i] - a[i]);
    }

    cout << ans << endl;
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int t = 1;
    // cin >> t;
    while(t--)
    {
        solve();
    }
    return 0;
}